Today's automobiles tend to have smaller, more demanding engines operating at higher temperatures. Thus, the engine oil has to function in an increasingly severe environment while meeting fuel economy demands. Besides changes in the additive package, increasingly synthetic base oils are being used instead of conventional mineral oils. Of the synthetic oils, poly alpha olefins (PAO) are among the most popular.
PAO is manufactured by the oligomerization of linear alpha olefins followed by hydrogenation to remove unsaturated bonds and fractionation to obtain the desired product slate. 1-decene is the most commonly used alpha olefin in the manufacture of PAO, but 1-dodecene and 1-tetradecene can also be used. PAO's are commonly categorized by the numbers denoting the approximate viscosity in centistokes of the PAO at 100.degree. C. It is known that PAO 2, PAO 2.5, PAO 4, PAO 5, PAO 6, PAO 7, PAO 8, PAO 9 and PAO 10 and combinations thereof can be used in engine oils. The most common of these are PAO 4, PAO 6 and PAO 8.
Conventionally, base oils of lubricating viscosity used in motor oil compositions may be mineral oil or synthetic oils of viscosity suitable for use in the crankcase of an internal combustion engine. Crankcase base oils ordinarily have a viscosity of about 1300 cSt at 0.degree. F. (-18.degree. C.) to 24 cSt at 210.degree. F. (99.degree. C.). The base oils may be derived from synthetic or natural sources. Mineral oil for use as the base oil in this invention includes paraffinic, naphthenic and other oils that are ordinarily used in lubricating oil compositions. Synthetic oils include both hydrocarbon synthetic oils and synthetic esters.
Although the common 1-decene based PAO 4, 6 and 8 offer better performance than mineral oil based engine oils, they encounter difficulties when subjected to the severe PV 1449, CEC L-78-T-96 and Sequence IIIE tests. The PV 1449 and Sequence IIIE tests evaluate fully formulated engine oils with respect to high temperature oxidative stability and piston deposits. The CEC L-78-T-96 test evaluates fully formulated engine oils with respect to piston cleanliness and piston ring sticking. The PV 1449 and CEC L-78-T-96 tests will be referred to hereinafter as the Volkswagen T-4 and TDI engine tests, respectively.
It has been found to be difficult to blend an engine oil of the desired 0W30 viscosity grade based on PAO 4 and 6 that successfully completes the TDI test. Repeatedly, it was found that too low oil pressure caused the engine to fail from 2 to 8 hours before the end of the test. In the T-4 test, it was found that the increase in engine oil viscosity resulting in engine failure during the test was related to oil oxidation stability and volatility. To pass the T-4 test, it was found that the PAO 4/6 based engine oil requires large quantities of expensive anti-oxidants. The other way to obtain PAO 4/6 based oil which passes the T-4 test is to use an expensive fully synthetic oil.
The Volkswagen T-4 and TDI tests have recently become an important measure of engine lubrication oil quality under very severe conditions. The Sequence IIIE test is analogous to a T-4 test but is specifically developed for U.S. built engines. The T-4 and Sequence IIIE tests are for gasoline engines and the TDI test is for diesel engines. They replicate the severe engine conditions put on motor lubrication oil by sustained, very high speed driving, as on the German Autobahn. What is needed is a PAO based oil which is able to successfully complete severe engine tests such as the Volkswagen T-4 and TDI tests and the Sequence IIIE test without having to use large quantities of anti-oxidants or a fully synthetic oil.
Surprisingly, it has been found that lubrication oils based on a feed consisting of 1-dodecene or 1-tetradecene, and that have approximate viscosities at 100.degree. C. of from 3.5 to 8.5 centistokes, successfully pass the T-4 and TDI tests with PAO based oil weight percentages much lower than previously achieved. This represents a major development in the search for an economical lubrication oil that is well suited for modern driving conditions.